Deformation and Fracture of Engineering Materials

The basic concern of semiconductor packaging  engineers is the proper design and assembly of IC packages so that these will not just meet their quality standards, but their reliability requirements as well.  More often than not, the failure of an IC package failure involves the deformation or fracture of one or more of its parts or features.  It is therefore important for assembly engineers to have a basic knowledge of why and how engineering materials deform and, in extreme cases, even fracture.

Deformation is the phenomenon in which a material undergoes changes in dimensions in response to mechanical forces.  The deformation is said to be elastic if the material returns to its original size and shape upon removal of the applied load. On the other hand, it is referred to as inelastic deformation if the application of the mechanical load results in a permanent change in the dimensions of the material, i.e., it doesn't return to its original size and shape even if the mechanical load is no longer being applied to it.

Fracture, or rupture, is the phenomenon wherein a structural component or feature breaks into two or more pieces. In the semiconductor industry, we all know that a package or any of its parts doesn't have to fracture before it becomes a failure.  Failure, which is defined as non-conformance to a specification, can occur once a deformation causes the package to not meet any of its specifications, with or without fracture.  This is why bent leads and warped packages are both considered as assembly failures.

Engineering stress, σ, is defined as the force applied per unit area.  Engineering strain, ε, on the other hand, is the ratio of the change in a dimension to its original value.  These definitions may be mathematically expressed as follows:    σ = F / Ao and ε = Δl / lo, where Ao and lo are the original cross-sectional area and length of the specimen subjected to a uniaxial force F, respectively, while l is the instantaneous length of the specimen.

For small values of strain during elastic deformation, the strain experienced by a specimen is linearly related to the stress applied on it.  This linear relationship between stress and strain is known as Hooke's Law.  The ratio of the stress to strain in the linear elastic region is known as Young's Modulus, E, which is also known as the elastic modulus.  Thus, E = σ / ε.  Young's modulus is a measure of the stiffness of a material, i.e., the higher the Young's modulus of a material, the stiffer it is, and the less strain it exhibits for a given stress.

The elastic limit of a material is the critical value that the applied stress needs to exceed for the deformation to become permanent.  If a material is loaded beyond its elastic limit, it can no longer go back to its original shape and size upon removal of the force.  Such a permanent deformation is also known as plastic deformation.  Once the plastic deformation region is reached, the stress-strain relationship ceases to be linear and no longer obeys Hooke's Law.

Materials increase in hardness upon experiencing plastic straining, a phenomenon known as strain hardening. In metals, strain hardening is due to interactions between dislocations in the metallic crystal, which significantly decrease the mobility of the dislocations. In polymers, strain hardening results from chain alignment in the stress direction.  Unlike metals and polymers, ceramics do not exhibit plastic deformation due to the restricted mobility of their dislocations, making ceramics brittle.

Another difference between elastic and plastic deformation is in how they change the shape and volume of a specimen.  A specimen that's under elastic deformation will change more in volume than its shape, since only the separation distance between atoms change, while retaining the atoms' nearest neighbors.  A specimen that's under plastic deformation, however, will change more in shape than in volume, since this type of deformation does not alter the bond lengths, but results in slip processes within its microstructure.